Mobile closing parts of vehicles, for example sliding roofs and window lifters, are equipped with a device for determining position. This device for determining position is necessary so that the windows and/or sliding roofs stop at designated positions. It is also necessary for ensuring the statutory requirements relative to anti-trap protection.
In cost-effective systems, the position is determined by counting the commutation current ripple. The commutation current ripple is counted by the current ripples, hereinafter referred to as the ripple, being removed by bandpass filtering. To achieve this, the current is usually rapidly sampled by an analog/digital converter, for example every 200 μsec, and then subjected to digital filtering. As the ripple frequency is determined by the motor rotational speed and this may vary widely, the center frequency of the bandpass filter used has to be correspondingly adjusted.
In this connection, there are two main approaches in the prior art. Either an adaptive bandpass filter is used, the coefficients thereof being altered. Alternatively, a plurality of fixed filters with variable center frequencies is used in combination with a multiplexer for selecting the filter which is best suited for the current motor rotational speed.
Both aforementioned solutions have the drawback that they require a high level of computing power. This, in turn, requires the use of a microcomputer with a high level of computing power and leads to high system costs.
It is known from the periodical “Elektronik”, No. 25 of 14.12. 1984, pages 71-72, to determine the rotational speed of a direct-current motor incrementally from the commutation ripple of the armature current and to use it for controlling the rotational speed. In this case an analog circuit determines an approximate measurement of the rotational speed from the motor current. Thus a voltage/frequency converter is activated so that its initial frequency sets the frequency limit of a switched capacitor filter, so that the fundamental component of the commutation ripple is present at the output thereof.
A method and a device for measuring the rotational speed of a mechanically commutated direct-current motor is disclosed in EP 0 689 054 A1. In this case, the ripple of the current arising during commutation is detected using an electronic circuit and the frequency of the ripple evaluated as a measurement of the rotational speed. For detection and signal conditioning, the measured object motor is coupled to the detection and measurement device via a direct-current path, and the generation of the current ripple for the detection and measurement device takes place via a direct-current path. The signal of the current ripple thus generated is phase-shifted. The phase-shifted and the non-phase-shifted signal of the current ripple are added to one another or subtracted from one another in analog form. The resulting current ripple signal is evaluated with regard to its frequency, this evaluation being carried out relative to its zero crossings.
A method and a device for measuring the speed of a direct-current motor is disclosed in EP 0 730 156 A1. In this case, the signal is evaluated at the negative terminal of the motor. This signal is high-pass filtered in order to separate the ac components of the signal. The frequency of this signal is in most cases proportional to the speed of the motor. This signal is fed to a comparator which converts it into a square wave signal. Said signal is fed to a microprocessor which determines the frequency thereof. If this is below a first predetermined threshold value or above a second predetermined threshold value, the microprocessor then switches the motor off and emits an error signal.
A method for determining the rotational speed in mechanically commutated direct-current motors from the time characteristic of the ripple of the motor current occurring during commutation is disclosed in EP 0 890 841 B1. In this method, the time characteristic of the ripple of the motor current is detected, the commutation time is determined and the rotational speed and/or the angle of rotation derived therefrom in an evaluation unit. At the same time as the detection of the ripple of the motor current from a motor state model, which is based on the electromechanical motor equations, a permissible reference time range is determined from the motor current and the motor voltage. The commutation times are taken into consideration by the evaluation unit only if said unit is in the permissible reference time range. If, within the permissible reference time range, no commutation ripple may be assigned, then the evaluation unit extrapolates a probable commutation time for this reference time range from the motor state model.